Method for determining the swelling clay content of cement mixtures by dielectric measurements

Abstract

A method for determining at wellsites the swelling-clay content of cement mixtures by high-frequency dielectric measurements including: grinding a cement mix sample to a size suitable for testing; washing the cement mix sample with a fluid having a water activity substantially less than water, which may contain a soluble cation; packing the washed cement mix sample into a sample cell suitable for dielectric measurement; measuring the dielectric constant at a preselected frequency; and comparing the measured dielectric constant of the cement mix sample to calibration curves.

We claim: 
     
       1. A method for measuring the swelling-clay content of wet cement mixtures by dielectric measurements, comprising the steps of: (a) washing a wet cement mix sample with a fluid having a water activity substantially less than that of water;   (b) packing the washed cement mix sample into a sample cell suitable for dielectric measurement;   (c) measuring the dielectric constant at a preselected frequency; and   (d) comparing the measured dielectric constant of the cement mix sample to a calibration curve to determine the swelling-clay content.   
     
     
       2. A method for measuring the swelling-clay content of cement mixtures by dielectric measurements, comprising the steps of: (a) grinding a cement mix sample to a size suitable for testing;   (b) washing the cement mix sample with a fluid having a water activity substantially less than that of water;   (c) packing the washed cement mix sample into a sample cell suitable for dielectric measurement;   (d) measuring the dielectric constant at a preselected frequency; and   (e) comparing the measured dielectric constant of the cement mix sample to a calibration curve to determine the swelling-clay content.   
     
     
       3. The method of claim 1 or 2 further including the step of repeating steps a through e for each sample to be compared. 
     
     
       4. The method of claim 1 or 2 wherein said fluid is a mixture of alcohol and water. 
     
     
       5. The method of claim 1 or 2 wherein said fluid is a mixture of 70% (volume) isopropyl alcohol and 30% (volume) water. 
     
     
       6. The method of claim 1 or 2 wherein said packing is accomplished by centrifuging the sample cell. 
     
     
       7. The method of claim 1 or 2 wherein the preselected frequency of measurement is about 1 megahertz. 
     
     
       8. A method for measuring the swelling-clay content of cement mixtures by high-frequency dielectric measurements, comprising the steps of: (a) grinding a cement mix sample to a predetermined size suitable for testing;   (b) washing the cement mix sample with a fluid having a water activity substantially less than that of water;   (c) packing the washed cement mix sample into a sample cell suitable for dielectric measurement;   (d) measuring the dielectric constant at a preselected frequency to allow uniform comparison of cement mix samples; and   (e) repeating steps a through d for a sample of pure swellable clay;   (f) repeating steps a through d for a sample of pure cement;   (g) obtaining the swelling-clay content of the cement mix in accordance with the following relationship: ##EQU2##  where: DCM cement  mix is the measured dielectric constant of the cement mix sample; DCM pure  cement is the measured dielectric constant of the pure cement sample;   DCM pure  swellable clay is the measured dielectric constant of te pure swellable clay sample; and   a is the amount swelling-clay content in the cement mix in a fraction of total weight.     
     
     
       9. The method of claim 8, further including the step of repeating steps a through g for each sample to be compared. 
     
     
       10. The method of claim 8 wherein said fluid is a mixture of alcohol and water. 
     
     
       11. The method of claim 10 wherein said mixture is 70% (volume) isopropyl alcohol and 30% (volume) water. 
     
     
       12. The method of claim 8 wherein said packing is accomplished by centrifuging the sample cell. 
     
     
       13. The method of claim 8 wherein the preselected frequency of measurement is about 1 megahertz. 
     
     
       14. A method for measuring the swelling-clay content of cement mixtures by high-frequency dielectric measurements, comprising the steps of: (a) grinding a dried cement mix sample to a predetermined size suitable for testing;   (b) washing the cement mix sample with a fluid having a water activity substantially less than that of water to which a cation has been added;   (c) packing the washed cement mix sample into a sample cell suitable for dielectric measurement;   (d) measuring the dielectric constant at a preselected frequency to allow uniform comparison of cement mix samples; and   (e) comparing the measured dielectric constant of the cement mix sample to a calibration curve derived from samples of known dielectric constant and swelling-clay content.   
     
     
       15. The method of claim 14 wherein said cation comprises Li + , Na + , K + , Rb + , Cs + , (NH 4 ) + , Mg +2 , Ca +2 , Ba +2 , Sr +2 , Co +2 , Ni +2 , Cu +2 , Zn +2 , La +3 , Fe +3 , Cr +3 , or Al +3 . 
     
     
       16. The method of claim 14 wherein said fluid is a mixture of alcohol and water. 
     
     
       17. The method of claim 16 wherein said mixture is 70% (volume) isopropyl alcohol and 30% (volume) water. 
     
     
       18. The method of claim 14 wherein said packing is accomplished by centrifuging the sample cell. 
     
     
       19. The method of claim 14 wherein the preselected frequency of measurement is about 1 megahertz. 
     
     
       20. The method of claim 14, further including the step of repeating steps a through e for each sample to be compared. 
     
     
       21. The method of claim 14, wherein the swelling-clay content of the cement mix is determined in accordance with the following relationship: ##EQU3## where: DCM cement  mix is the measured dielectric constant of the cement mix sample; DCM pure  cement is the measured dielectric constant of the pure cement sample;   DCM pure  swellable clay is the measured dielectric constant of te pure swellable clay sample; and   a is the amount swelling-clay content in the cement mix in a fraction of total weight.